Electrically conductive, thermoplastic, heat-activated adhesive film

ABSTRACT

Electrically conductive, thermoplastic and heat-activatable adhesive film, comprising
     i) a thermoplastic polymer in a proportion of from 30 to 89.9% by weight,   ii) one or more tackifying resins in a proportion of from 5 to 50% by weight and/or   iii) epoxy resins with hardeners, with or without accelerators, in a proportion of from 5 to 40% by weight, and   iv) silver-coated glass beads or silver particles in a proportion of from 0.1 to 40% by weight.

The invention describes an electrically conductive, thermoplastic andheat-activatable adhesive film as used for the permanent connection oftwo articles.

Electronic components are becoming ever smaller, which makes theirhandling and processing ever more difficult. Especially when producingelectrical contacts between the components and/or the connections, it isfound that conventional soldering can no longer provide simple andcost-effective connection of the corresponding contacts.

The adhesive bonding of electronic components by means of electricallyconductive adhesive films is therefore being developed as analternative.

For the field of electrically conductive adhesive tapes, it is prior artto employ conductive pigments such carbon black, metal powders, ioniccompounds and the like in adhesive compositions.

In sufficient quantities, the particles contact one another and thepossibility of current flow from particle to particle is provided. Thecurrent flow here is not directionally oriented (isotropic); forspecific applications, such as electronic switches, contacting ofconductors, etc., there is, however, the requirement to achieveelectrical conductivity only in the depth direction (z direction)through the adhesive tape, with no conductivity, however, in thetwo-dimensional extent (x-y plane) of the adhesive film.

In special cases, it is also required/must be ensured that theconductive sites through the adhesive film (in the z direction)

-   -   are distributed homogeneously, so that any sites on the adhesive        tape can be used identically and lead to the same results;    -   have small cross sections, so that even in the electronics        sector conductor tracks lying close together can be selectively        connected without the risk of short circuits, and    -   are insulated from one another, in that the areas in between are        not filled with conductive materials.

The U.S. Pat. No. 3,475,213 describes randomly distributed sphericalparticles which consist entirely of a conductive metal or are providedwith an electrically conductive layer. The best results are achievedwith particles which are only slightly smaller than the thickness of thefilm of adhesive composition.

In U.S. Pat. No. 5,300,340, a special production method using a rotatingdrum is employed to site the electrically conductive particles in theadhesive composition.

Both of the pressure-sensitive adhesive (PSA) tapes described are basedon self-adhesive acrylate polymer compositions and are unable to connecttwo substrates with the strength required for a permanent bond.Connections subject to permanent or repeated stress, such as by tensile,torsional or shear forces, for example, exhibit signs of detachmentafter just a short time. The reason for this is that, in the cited priorart, the generally low bond strength of the PSA tapes is reduced furtherby the addition of electrically conducting particles. The joiningtechniques are therefore inadequate for ensuring a permanent bond in thecase of electronic contacts which are subject to mechanical stress.

The particles that are added firstly reduce the bond strength andsecondly increase the distance from the adhesive tape to the surface,since the particles project from the surface to a certain extent, whichis entirely desirable for improving the electrical conductivity.

In the case of a product structure as described by U.S. Pat. No.5,300,340, this feature is deliberately exploited by mixing inrelatively large particles having a diameter which is greater than thethickness of the adhesive composition.

Not only do the processes presented above display inadequate bondstrengths for electrical contacts subject to mechanical stress, thebonds produced with them can also be parted again, as a result of whichmanipulations are possible and property rights, especially in the caseof sensitive electronic components, can easily be infringed.

Particularly in the case of electronic devices which are small andflexible and are used in an electronic toy or in chip cards, theelectrically conducting adhesive bond is frequently not protected by arigid housing, is required to withstand flexural stresses withoutloosing the electrical contact, and is susceptible to such manipulation.

The object of the invention is to achieve an effective and permanentbond while simultaneously producing an electrically conductive contactin the case where a carrier element in data carriers or electroniccomponents is bonded with the aid of a heat-activatable adhesive film.

This object is achieved by means of an adhesive film as characterizedfurther in the main claim. The subclaims relate to advantageousdevelopments of the subject matter of the invention,

In accordance with the invention, the electrically conductive,thermoplastic and heat-activatable adhesive film comprises

-   i) a thermoplastic polymer in a proportion of from 30 to 89.9% by    weight,-   ii) one or more tackifying resins in a proportion of from 5 to 50%    by weight and/or-   iii) epoxy resins with hardeners, with or without accelerators, in a    proportion of from 5 to 40% by weight, and-   iv) silver-coated glass beads or silver particles in a proportion of    from 0.1% by weight, very preferably 10% by weight, to 40% by    weight.

The adhesive film is a mixture of reactive resins, which crosslink atroom temperature and form a three-dimensional polymer network of highstrength, and permanently elastic elastomers, which act againstembrittlement of the product. The elastomer may preferably be from thegroup consisting of polyolefins, polyesters, polyurethanes andpolyamides or can be a modified rubber, such as nitrile rubber, forexample.

The particularly preferred thermoplastic polyurethanes (TPUs) are knownreaction products of polyester polyols or polyether polyols and organicdiisocyanates such as diphenylmethane diisocyanate. They are composed ofpredominantly linear macromolecules. Such products are generallyobtainable commercially in the form of elastic granules—for example,from Bayer AG under the trade name “Desmocoll”.

By combining TPU with selected compatible resins, it is possible toachieve a sufficient reduction in the softening point of the adhesivefilm, so as to prevent deformation of the card body during theproduction process. In parallel with this, there is, in fact, anincrease in the adhesion. Examples of resins which have been foundsuitable are certain rosins, hydrocarbon resins and coumarone resins.

Alternatively, the reduction in the softening temperature of theadhesive film can be achieved by combining TPU with selected epoxyresins based on bisphenol A and/or F and a latent hardener. An adhesivefilm comprising such a system permits aftercuring of the joint, eithergradually at room temperature without any further external intervention,or briefly by controlled heat treatment of the cards followingproduction. In this way, it is possible to prevent subsequentlynon-destructive removal of the chips with criminal intent using, forexample, a standard iron.

The chemical crosslinking reaction of the resins brings about highstrengths between the adhesive film and the surface to the bonded andachieves a high internal strength in the product.

The addition of these reactive resin/hardener systems also leads to areduction in the softening temperature of the abovementioned polymers,which has the advantageous effect of reducing their processingtemperature and processing rate. The suitable product is a product whichis self-adhesive at room temperature or slightly elevated temperatures.When the product is heated, there is a short term reduction in theviscosity, as a result of which the product is able to wet roughsurfaces as well.

The beads present in the adhesive film permit conductivity only in the zdirection; in the x-y plane, owing to the lack of mutual contact, thereis not conductivity.

The compositions for the adhesive film can be varied widely by alteringthe nature and proportion of the raw materials. Similarly, it ispossible to obtain further product properties, such as colour, thermalconductivity or electrical conductivity, for example, by targetedadditions of colorants, mineral and/or organic fillers, and/or powdersof carbon and/or metal.

The adhesive film preferably has a thickness of from 20 to 500 μm.

The silver particles may consist of pure silver or else may be producedfrom an alloy, which in that case should contain a considerableproportion of silver in order to ensure conductivity.

References below to (silver-coated) glass beads will be understood bythe person skilled in the art as embracing the abovementioned silverparticles.

In a first, advantageous embodiment of the invention, the diameter ofthe silver-coated glass beads is at least equal to the thickness of theadhesive film and may even be somewhat greater than the thickness of theadhesive film to be produced.

In an alternative, advantageous embodiment of the adhesive film, thediameter of the glass beads is between 10 and 20 μm smaller than thethickness of the adhesive film.

Which diameter of the glass beads is chosen in accordance with theinvention depends on the particular intended use of the adhesive film.

If the diameter of the glass beads is greater than the adhesive-filmthickness, glass beads projecting from the adhesive film may lead tounwanted air inclusions in the joint, which may reduce the bondstrength. Under adverse conditions, this may result in the glass beadsin an elastic joint losing contact under mechanical stresses, which canbe reestablished only by repeating the pressing operation.

In the case of some applications, therefore, the bond strength is moreimportant than the conductivity. In these cases, bonding must beconducted at high pressure and at elevated pressure. In this specificcase, it is possible to omit electrically conductive glass beadsprojecting from the adhesive film.

The conductive glass beads in this case can be about 10 to 20 μm smallerthan the thickness of the adhesive film, so permitting easy attachmentand full-area bonding without air inclusions.

Despite this, electrical contact is produced, since under these bondingconditions the viscosity of the adhesive composition is reduced soseverely that it is displaced and the thickness of the joint is reduced.This takes place, for example, when adhesively bonding modules in smartcards. In this case, a pressure of 60 N per module and a die temperatureof about 200° C. are chosen. Under these conditions, the conductiveglass beads receive an electrically conductive contact, since theadhesive composition is displaced and is able to escape into a cavitybelow the chip module.

This is important, for example, when implanting a module in a cardhaving an antenna in the card body.

The adhesive film of the invention can be employed with particularadvantage to implant electrical modules into a card body which isprovided with a cutout which is intended to accommodate an electronicmodule having a plurality of contact surfaces on the first side andhaving on the second side, which is opposite to the first side, an ICchip whose terminals are connected via electrical conductors to thecontact surfaces, the adhesive film being used to connect the secondside of the module to the card body.

Preferably, in this case, the adhesive film has the same dimensions asthe module and is in the form of a punched film section.

Furthermore, it is also possible to use the adhesive film for structuralbonding, with or without subsequent heat-curing.

The invention describes an electrically conductive, thermoplastic andheat-activatable adhesive film as used for the permanent connection oftwo articles. In contrast to adhesive bonds produced with a PSA tape,strengths as required in the construction sector are permanentlyachieved and are retained even under chemical, thermal or climaticstress.

To produce the adhesive film, the composition forming the film is castas a solution onto a flexible substrate (release film or release paper)and is dried, so that the composition can easily be removed again fromthe substrate.

Following appropriate processing, punched sections or rolls of thisadhesive film can be bonded to the adherend substrate (electroniccomponent, module, etc.) at room temperature or at slightly elevatedtemperature.

The admixed reactive resins should not enter into any chemical reactioneven at the slightly elevated temperature. Therefore, the bondingoperation need not be carried out as a one-stage process; rather, forthe sake of simplicity, it is possible, as with a PSA tape, first toattach the adhesive film to one of the two substrates by lamination atthe elevated temperature. Then, in the actual process of hot bonding tothe second substrate, the resin cures partly or fully and the bondedjoint attains the high bond strength, far above that of PSA systems.

Accordingly, the adhesive film is particularly suitable for hot pressingat temperatures below 120° C., in particular from 80 to 100° C.

In contrast to adhesive pastes or liquid adhesives comprising conductivefiller, which are usually suitable only for isotropically conductiveconnection, however, the adhesive film described does not cure to abrittle film, but instead, owing to the balanced proportion ofcrosslinker resin and elastic rubber, remains in a viscoelastic state,as a result of which it is able effectively to withstand peel movementand peel stresses in particular. The great advantage of the adhesivefilm described comes to bear wherever an adhesive bond or fastening andan electrically conductive connection have hitherto been carried out intwo separate steps. In the great majority of cases, this also means anincrease in the space required for fastening and conductive connection,which in the case of relatively small electronic components is adisadvantage. Furthermore, the separate bonding step requires specialequipment and expensive machinery.

The adhesive films of the invention are therefore distinguished by anumber of advantages:

-   -   They possess high cohesion and elasticity at room temperature.    -   They exhibit high adhesion to the conventional chip card        materials such as PVC, PC, PET or ABS, for example.    -   They can be activated at below the softening temperature of the        card materials.

Furthermore, chip cards whose modules are bonded in with an adhesivefilm of the invention feature particularly high flexural strength.

This is demonstrated by conducting a long-term flexural test under aconstant load cycle in accordance with DIN EN 20 178.

In the text below, a number of examples illustrate the adhesive film ofthe invention without wishing to restrict the described inventionunnecessarily.

EXAMPLE 1

The following constituents were dissolved in an acetone/methyl ethylketone mixture, applied as a solution to a siliconized paper, and thendried.

Trade name % by weight Thermoplastic PU (TPU) Desmocoll 400 55 Epoxyresin (Bisphenol A) Rütapox 0164 25 Dicyandiamide Dyhard 100 S 5 (SKWTrostberg) Silver-coated glass beads Conductofil 20-60 15 Thickness ofthe dried μm 58 ASTM D 1000 adhesive film Weight of the adhesive filmg/m² 55 ASTM D 1000 Contact resistance mΩ 3.5 ASTM D 2739 Specificresistance Ωm 0.30 ASTM D 2739 Bond strength N/mm² 10 DIN EN 1465

EXAMPLE 2

The following constituents were dissolved in an acetone/methyl ethylketone mixture, applied as a solution to a siliconized paper, and thendried.

Substance class % by weight Nitrile rubber 55 Phenolic resin 29.8 Ageinginhibitor 0.2 Silver-coated glass beads 15 Thickness of the dried μm 59ASTM D 1000 adhesive film Weight of the adhesive film g/m² 55 ASTM D1000 Contact resistance mΩ 3.5 ASTM D 2739 Specific resistance Ωm 0.32ASTM D 2739 Bond strength N/mm² 7.0 DIN EN 1465

1. Electrically conductive, thermoplastic and heat-activatable adhesivefilm, comprising i) a thermoplastic polymer In a proportion of from 30to 89.9% by weigth, ii) a) one or more tackifying resins in a proportionof from 5 to 50% by weight or b) epoxy resins with hardeners, with orwithout accelerators, in a proportion of from 5 to 40% by weight, or c)both said one or more tackifying resins in a proportion of from 5 to 50%by weight and said epoxy resins with hardeners, with or withoutaccelerators, in a proportion of from 5 to 40% by weight, and iii)silver-coated glass beads in a proportion of from 0.1 to 40% by weight,iv) where the diameter of the glass beads is at least equal to thethickness of the adhesive film.
 2. Adhesive film according to claim 1,wherein the thermoplastic polymer comprises a member selected from thegroup consisting of thermoplastic polyolefins, polyesters, polyurethanesor polyamides and modified rubbers.
 3. The adhesive film of claim 2,wherein said modified rubbers are nitrile rubbers.
 4. Adhesive filmaccording to claim 1, wherein the adhesive film is blended with one ormore additives.
 5. The adhesive film of claim 4, wherein said additivesare selected from the group consisting of colorants and mineral ororganic fillers.
 6. The adhesive film of claim 5, wherein sold additivesare selected from the group consisting of silica, carbon powders, andmetal powder.
 7. Thermoplastic adhesive film according to claim 1,wherein the adhesive film has a thickness of from 20to 500 μm. 8.Thermoplastic adhesive film according to claim 1, wherein the adhesivefilm is not pressable at temperatures below 120° C.
 9. The adhesive filmof claim 8, wherein said temperatures are from 80° C. to 100° C. 10.Thermoplastic adhesive film comprising i) a thermoplastic polymer in aproportion of from 30 to 89.9% by weight, ii) a) one or more tackifyingresins in o proportion of from 5 to 50% by weight or b) epoxy resinswith hardeners, with or without accelerators, in a proportion of from 5to 40% by weight, or c) both said one or more tackifying resins in aproportion of from 5 to 50% by weight and said epoxy resins withhardeners, with or without accelerators, in a proportion of from 5 to40% by weight, and iii) silver-coated glass beads In a proportion offrom 0.1 to 40% by weight, iv) where the diameter of the glass beads isat least equal to the thickness of the adhesive film, and wherein theadhesive film is in the form of a punched film section.
 11. A method forImplanting electrical modules in a card body provided with a cutout foraccommodating an electronic module which on a first side has a pluralityof contact surfaces and on a second side, which is opposite the firstside, has an IC chip whose terminals are connected via electricalconductors to the contact surfaces, wherein an electrically conductive,thermoplastic and heat-activatable adhesive film, comprising i) athermoplastic polymer in a proportion of from 30 to 89.9% by weight, ii)a) one or more tackifying resins in a proportion of from 5 to 50% byweight or b) epoxy resins with hardeners, with or without accelerators,in a proportion of from 5 to 40% by weight, or c) both said one or moretackifying resins in a proportion of from 5 to 50% by weight and saidepoxy resins with hardeners, with or without accelerators, in aproportion of from 6 to 40% by weight, and iii) silver-coated glossbeads In a proportion of from 0.1 to 40% by weight, iv) where thediameter of the glass beads is at least equal to the thickness of theadhesive film is used to connect the second side of the module to thecard body.